A level shifter is a circuit that converts an input signal having the voltage amplitude of VDD1 into an output signal having the voltage amplitude of VDD2. Typically, the amplitude is converted by fixing the lower potential side and converting the potential at the higher potential side. Level shifters are widely used in integrated circuits where more than one type of circuit, each with different operating voltage amplitudes, are integrated together.
A common application for a level shifter is shifting the voltage of a signal transferred between an IO circuit and a core circuit in an integrated circuit device. Traditionally, the IO operating voltage was compatible with the core operating voltage. However, with the shrinking of VLSI circuits, the operating voltage of core circuits has steadily been lowered, while the IO operating voltage has stayed relatively steady, so that currently, core operating voltage is typically much lower than IO operating voltage. For example, in deep micron technology, the core operating voltage has dropped to about 0.9 to 1V for a state of the art device, while the IO operating voltage is typically about 3.3V to 5.0V. Therefore, a signal needs to be level shifted (up) before it is sent from a core circuit to an IO circuit or (down) from an IO circuit to a core circuit.
FIG. 1 illustrates a conventional cross-latch level shifter circuit that converts an input signal with amplitude of VDD, which is, e.g., a core operating voltage, to a signal with amplitude of VDDIO, which is, e.g., an IO operating voltage that is higher than VDD. Node 2 is a power supply node at a power voltage VDDIO. Node 4 is at VSS, typically ground potential or 0V. Node 6 is an input node and node 8 is an output node. The input signal voltage at node 6 switches between 0V to VDD. If the input signal at input node 6 is 0, the nMOS transistor 14 is off, so that its drain voltage at node 24 is high. Inverter 18, whose input is low (0V) at node 6, outputs a high level (VDD) on its output, which is tied to node 22. Because node 22 is high, nMOS transistor 16 is on, thus pulling the output voltage at node 8 low. Conversely, if the input signal at the input node 6 is VDD, the nMOS transistor 14 is on so that its drain voltage 24 is low. The voltage at node 22 is converted to low by the inverter 18 so that the nMOS transistor 16 is off, and the output voltage at node 8 is pulled up by pMOS transistor 12 to VDDIO. Therefore, the input signal with amplitude of VDD is shifted to VDDIO.
The circuit illustrated in FIG. 1 performs well when the VDD is higher than the threshold voltage of the transistors 14 and 16 with an adequate margin. However, this circuit has several disadvantages. First, since the power supply voltage VDDIO is high, the transistors 10, 12, 14 and 16 are typically thick oxide transistors so that they can withstand higher operation voltages without being damaged. The thick oxide transistors have higher threshold voltages. To turn a thick oxide transistor 14 or 16 from off to on, the input signal at node 6, or its inverted signal at node 22, must rise higher than the threshold voltage. The circuit state transition is slow due to the high threshold voltages of the transistors 14 and 16. Second, with the scaling of the VLSI circuit, the gate oxide of the core CMOS gets thinner. The supply voltage VDD of the core CMOS is also lowered to protect the gate oxide from damage and the hot carrier effect. When VDD is dropped to equal to or lower than the threshold voltage of nMOS 14 and 16, the input signal with amplitude of VDD is not high enough to turn on the transistors 14 and 16, and the conventional low-to-high level shifter fails.
The circuit illustrated in FIG. 1 has been modified in the art to solve the above-described shortcomings. FIG. 2 illustrates a typical modified circuit. The input signal at node 6 is coupled to a thin oxide nMOS transistor 36 and the inverted input signal is coupled to a thin oxide nMOS transistor 38. If the drains of transistors 38 and 36 are directly connected to nodes 8 and 24, respectively, the circuit may work, but the thin oxide transistors 36 and 38 may be damaged since their drain voltage may be as high as VDDIO. A voltage divider circuit is added to divide the voltages applied to transistors 36 and 38 so that transistors 36 and 38 always work under a desired operation voltage. The voltage divider circuit is composed of transistors 26, 28, 30, and 32. Transistors 26 and 28 are native transistors. Native transistors sit in a wafer substrate or in wells of the same type as the substrate. They typically have low threshold voltages that are close to zero volts. In this circuit, native transistors 26 and 28 have negative threshold voltage, so that they are always on even when the input signal or inverted input signal applied on their gates is 0 volts. Transistors 30 and 32 are biased at VDD. The transistors 26, 28, 30 and 32 have voltage drops between their sources and drains. The design of the circuit ensures that the drain voltages of the transistors 36 and 38, which are VDDIO minus the voltage drops on transistors 26, 28, 30, and 32, are close to the core operation voltage VDD. Therefore, thin oxide transistors 36 and 38 can be used in the level shifter to improve circuit performance without compromising circuit reliability.
However, the process of making the level shifter shown in FIG. 2 has more steps, and production cost is higher since there is an extra type of device to be formed. The native transistors 26 and 28 have higher leakage current than the normal transistors. They also take up more space and their size does not shrink in correspondence with the shrinking of the size of the integrated circuit, so the leakage current will be more significant.
What is needed, therefore, is a level shifter that overcomes the above described shortcomings in the prior art.